A widespread problem in the production of optoelectronic components is that, after epitaxial growth of the semiconductor layer on a growth substrate, a plurality of photolithographic steps have to be performed on the p-side, i.e. from the side which is subsequently provided with the p-contact, before the growth substrate is separated from the epitaxial layer for example by laser lift-off (LLO).
In laser lift-off (LLO) the substrate, which is to be detached from the other layers, is irradiated with a pulsed or a non-pulsed laser, which penetrates the substrate. This results in heating of the surface of the semiconductor layer, which adjoins the substrate. When the required temperature is reached the substrate is split off.
A method of separating substrate and semiconductor layer by means of laser lift-off is explained for example in DE 19640594 A1, the disclosure content of which is hereby included in this respect by reference. Method steps prior to the LLO may hereinafter be named as “pre-LLO”, and those thereafter as “post-LLO”.
In semiconductor manufacturing, the term “mesa” is used to denote a protected region raised after an etching step. The recesses obtained by the etching process are known as “mesa trenches”, and the side walls of the mesa trenches as “mesa flanks”. Each etching step is usually preceded by a photolithographic step, in which the region which is intended subsequently to form the mesa is protected from the etching chemical by means of a photoresist. To this end, the entire surface is first coated with a photoresist, then a mask is placed onto the resist, and the regions not covered by the mask are exposed to light. As a result of exposure to light, these sub-regions of the resist cure, whereas the unexposed regions may be washed away in the next step. The regions not protected by the cured photoresist can be etched in a further method step.
In a widespread production method, four photolithographic steps have to be performed prior to LLO. One of these includes GaN mesa etching from the p-side, while the n-side is joined to the growth substrate. After detachment of the growth substrate by LLO, a further photolithographic step is generally necessary to form the n-contact structure.